The invention relates to a capacitive charge pump circuit employing complementary bipolar transistors and complementary field effect transistors (BiCMOS) capable of ensuring a substantial multiplication of the supply voltage also when it is exceptionally low.
Capacitive charge pump circuits are widely used in electronic systems for generating voltages higher than the supply voltage. The capacitive charge pump circuit is basically a voltage boosting circuit that may be used either alone as a voltage doubler or as a stage or cell, to be connected in cascade to other similar cells, for realizing multi-stage voltage multipliers.
Basically, as shown in FIG. 1, a capacitive charge pump circuit is composed of a charge transfer (pumping) capacitor C1, connected to the intermediate node of a pair of diodes D1 and D2, connected in series between a supply (input) node and an output node, to which a storage capacitor C2 is connected. A pair of switches SW1 and SW2, driven in phase opposition to each other and at a certain frequency, switch the charge transfer capacitor alternatively to ground potential (charge phase) and to the supply node (charge transfer phase). If we neglect the voltage drop on the diodes D1 and D2, it may be seen that the circuit is theoretically capable of providing an output voltage V.sub.OUT which is twice the supply voltage V.sub.S. (However, this ideal result is an approximation which assumes that: the voltage drop on the diodes D1 and D2 is negligible; the on-state voltage drop of the driving switches SW1 and SW2 is negligible; and the load current is negligible. In practice none of these assumptions is wholly accurate.)
More precisely, the maximum open circuit output voltage that can be obtained is given by: EQU V.sub.OUT =2V.sub.S -2V.sub.diode -V.sub.SW1 -V.sub.SW2.
With a relatively high supply voltage V.sub.S, the above noted voltage drops may be reasonably neglected. Certainly this condition hardly exists in low voltage, battery powered systems. In portable apparatuses, as for example in electrical wrist watches, hearing aids, sensors and similar the electronic circuits, displays and actuators are often powered with small batteries at a nominal voltage that may often be comprised between 1.2V and 1.4V. With almost exhausted batteries, the supply voltage may even drop toward 1.0V.
In these conditions, it is evident that the above noted voltage drops on the circuit components that form a capacitive charge pump circuit assume great importance up to make critical the operation of the circuit itself.
If the circuit is realized with a "bipolar technology", that is by employing bipolar transistors as shown in FIG. 2, the switches may be implemented in the form of a complementary pair of bipolar transistors T1 and T2. In view of the fact that their offset voltage is typically of about 0.7V (V.sub.BE =0.7V), they can ensure switching also with a supply voltage of about 1V.
On the other hand, the voltage drop on the diodes D1 and D2 and on the driving switches themselves (T1 and T2) do not allow, with a supply voltage in the order of 1-1.5V, a substantial duplication of the output voltage V.sub.OUT and the efficiency of the circuit drops drastically.
In order to obviate this typical limitation of a charge pump circuit made with bipolar components, it is known to realize the circuit with field effect devices, for example with MOSFETs, that is by realizing the integrated circuit with a CMOS technology, as shown in FIG. 3.
The diodes D1 and D2 of the functional circuit of FIGS. 1 and 2 are substituted by MOS transistors M3 and M4 which virtually form a "synchronous rectifier", being the MOS transistor virtually free of offset.
By realizing the driving switches with a CMOS pair of transistors M1 and M2, the circuit is theoretically capable of producing an output voltage V.sub.out which is twice the supply voltage V.sub.S.
This alternative solution, which is certainly advantageous in terms of "yield", has the drawback that with an exceptionally supply voltage, the circuit may cease to work by failing to switch. In fact, since the threshold value of the complementary pair of transistors M1 and M2 is strongly dependent on the temperature of operation, a supply voltage of at least 1.2-1.3V may be required for ensuring the switching of the driving switches M1 and M2.
In many apparatuses, it is important to ensure a correct operation even under precarious charge conditions of the battery (approaching exhaustion), that is with a supply voltage that may drop into the vicinity of about 1.1-1.0V and clearly, in these applications a CMOS circuit as the one depicted in FIG. 3, cannot be employed because it would make critical the operation of the circuit at exceptionally low supply voltages.
It is the main object of the invention to provide a capacitive charge pump circuit particularly suited for low supply voltage applications, capable of ensuring a correct operation even with a supply voltage or input voltage well below the limit of correct operability of a CMOS circuit, though being capable of producing a substantial doubling of the supply voltage even under such critical supply conditions.
This objective is fully met by the circuit of the invention that can be realized with a mixed fabrication technology (BiCMOS), and which employs bipolar transistors and field effect transistors.
Basically the circuit of the invention has a "bipolar portion" intrinsically capable of ensuring switching even with a supply (or input) voltage close to or equal to 1.0 Volt, and a CMOS portion, capable of practically nullifying (or at least markedly reducing) the voltage drops through the bipolar components of the circuit and therefore permitting a rise of the boosted output voltage close to a theoretically doubling of the supply (input) voltage.
In practice, the bipolar part of the circuit ensures operability of the circuit when it is first switched on, by producing a rise of the boosted output voltage, whenever the supply (or input) voltage is below the minimum value for a correct functioning of the CMOS part of the circuit. The substantial rise of the output voltage over the actual supply voltage that is eventually guaranteed by the bipolar part of the circuit, is sufficient to permit switching in the CMOS part of the circuit, which by intervening to essentially eliminate voltage drops, permits the circuit as a whole to reach a steady state working condition whereby it produces a substantial doubling of the supply (or input) voltage.
Essentially the bipolar part of the charge pump circuit of the invention is different from a typical charge pump bipolar circuit because the first diode (D1 of FIGS. 1 and 2) is replaced by a bipolar transistor driven by a bipolar stage controlled by the switching (clock) signal.
The CMOS part of the circuit of the invention essentially consists of a first field effect transistor connected in parallel and driven in phase with the bipolar transistor that constitutes the switch toward ground of the charge transfer capacitor and by a second field effect transistor, functionally connected in parallel with the charge diode of the output capacitor, driven in phase with the charge transfer from the charge transfer capacitor to the output storing capacitor.
The CMOS pair of transistors is driven by the switching (clock) signal through a level shifting bipolar stage followed by an inverting stage, both powered with the voltage present on the output node of the charge pump circuit. The second inverting stage may be realized by a CMOS inverter, though it may also be realized by a second bipolar stage.
As soon as the voltage present on the output node of the circuit reaches and becomes higher than the minimum voltage for overcoming the turn-on threshold of the MOS transistor or of the MOS transistors that form the second inverting stage, or, in case that also the second inverting stage is bipolar, the turn-on threshold of the CMOS pair of transistors, functionally connected in parallel with the respective bipolar elements, these CMOS transistors begin to switch, practically eliminating the relative voltage drops.
The operation of the circuit under critical conditions of the input voltage, for example when switching on the circuit and/or in presence of a battery close to exhaustion, is ensured by the bipolar part of the capacitive charge pump circuit, which is capable of rising the voltage on the output mode above the supply or input voltage by an amount sufficient to drive the CMOS part of the circuit which thereafter ensure a substantial duplication of the supply voltage (or of the input voltage in case of a multi-stage voltage multiplier). In case of a multi-stage voltage multiplier circuit, the basic BiCMOS circuit (cell or module) of the invention may be effectively employed as a first stage. The other stages of the voltage multiplier may be realized entirely with a CMOS technology (for example as shown in FIG. 3).